Prefer backpressure over snapshot guarantees

[vweevers]

Continuing from #68.

TBD if this should go in v3.

[vweevers]

I'm not sure whether to include this in v3 or exactly how to implement it. It can also be added in a minor later, but the thing that bothers me is that the readme states "Store large amounts of data in modern browsers". You can write large amounts of data, sure, but without backpressure, you're gonna run into trouble when reading.

[vweevers]

@ralphtheninja thoughts?

[ralphtheninja]

I'd skip it for v3 so we can get level-browserify out and later level. Then we can work on the minor as you mentioned.

[vweevers]

Relevant: Level/community#63 (comment) and Level/community#63 (comment). IMO offering proper backpressure is a higher priority now.

[vweevers]

I now prefer that we forgo snapshotting guarantees, in favor of backpressure and avoiding a blocking transaction.

[vweevers]

Plan:

• Implement iterator.all() with snapshot guarantees. This offers a nice alternative.
  • See API design: getMany(), key & value iterators, iterator.nextv() abstract-leveldown#380
• Drop snapshot guarantee from (regular use of) iterators